Telecommunication nodes in a network maintain synchronization of their docks by use of a single reference clock within the network. The reference clock transmits time-of-day information and/or a reference signal with a specified frequency. If a connection between a sub-network and the reference clock fails, one of the nodes in the sub-network enters “holdover”. This node uses its own local oscillator to maintain the last frequency/time to the best of its ability. It is important that the accuracy of the clocks of the sub-network relative to the reference clock, to which a connection is no longer possible, be maintained as high as possible. In this way the time knowledge of the sub-network matches the time knowledge of other parts of the network, so that handoffs to other sub-networks are possible for example.
However all oscillators drift in frequency because of environmental conditions, such as the temperature of the oscillator, and aging characteristics. One solution to maximizing the accuracy of the holdover in a sub-network is to use an expensive oscillator in the node which enters the holdover for the sub-network. More expensive oscillators usually have less drift. However this increased expense of the oscillator is not always practical, especially when there are many possible sub-networks such as in small cells, metrocells, and cell site routers. These more expensive oscillators usually also occupy more space within the node, which may be undesirable.
Another solution, which may be used together with a more expensive oscillator, is to control the ambient temperature of the oscillator so as to reduce drift. These are termed oven controlled oscillators or double over-controlled oscillators. However such control of temperature also requires more space and is more expensive than oscillators which are not oven controlled.
Yet another solution is to install a GPS receiver at the node responsible for holdover. While accurate maintenance of the clock is possible using a GPS receiver, the addition of a GPS receiver to a node adds to the cost of the node.